Aperture synthesis radiometry is becoming a feasible concept for imaging applications, especially at low microwave frequencies where it takes clear advantage of the absence of mechanical antenna motion. A 2D interferometric radiometer consists of a large number of receivers with small antennas distributed along a 2D structure, and the brightness temperature image is formed by inversion of the measured cross-correlation between all pairs of antennas. This is the concept of MIRAS (MIcrowave Radiometer by Aperture Synthesis), the core instrument of the SMOS (Soil Moisture and Ocean Salinity) mission selected by the European Space Agency (ESA) and planned to be launched in 2005. In its preliminary design, MIRAS receivers are uniformly distributed along a Y-shape structure and work at L-band. This approach, however, poses a challenge in the specifications required for the receivers: a) The short integration time due to the platform motion strongly limits the achievable sensitivity, b) the spatial resolution is determined by the structure dimensions which cannot be made arbitrarily large and c) the radiometric accuracy depends on the non ideal behavior of the receivers, although, to some extent can be corrected by internal calibration. This paper contributes to define the main trade-off between hardware requirements and system performance of this complex instrument.